Surface treated steel sheets having double coatings, metallic chromium and chromium oxide coatings are designated tin-free steel of chromium type (to be simply referred to as TFS, hereinafter). TFS is regarded as a substitute for tin plates because of its improved properties as can-forming material, and the demand for TFS is increasing in these years. Since TFS has metallic chromium and chromium oxide coatings on the surface, it does not possess sufficient weldability. In manufacturing cans from TFS, a can barrel is formed by applying an epoxy-phenol resin to a blank and bonding the blank with a nylon adhesive.
Recently, the extent of application of TFS cans has been further spread. That is, TFS cans are not only used for so-called cold packs prepared by filling cans with contents such as carbonated beverage and beer at low temperatures, but also used for so-called hot packs prepared by filling cans with contents such as fruit juice and coffee at high temperatures for sterilization as well as retort packs requiring a high temperature retorting treatment for sterilization at the end of packing. In the latter applications, there often occurred accidents of rupture of can barrels.
This can barrel rupture occurs in bonded TFS cans during hot packing and retorting treatment because hot water penetrates through the lacquer film at the barrel junction to deteriorate the interfacial adhesion between the lacquer film and the TFS sheet to eventually separate the lacquer film from the TFS sheet. Extensive investigations have been made to develop TFS sheets which are not susceptible to deterioration of the adhesion between the lacquer film and the TFS sheet.